1. Field of the Invention
This invention relates to a singlemode laser source tunable in wavelength having an external cavity.
2. Background of the Invention
It is known that an optical cavity resonated by a laser source selects one or more wavelengths emitted by a laser amplifier medium. This cavity frequently comprises two mirrors, one of which is partially transparent, forming what is known as a Fabry-Perot cavity. Such a Fabry-Perot cavity selects, or resonates for half-wavelengths equal to sub-multiples of the cavity's optical length L.sub.op, and which are therefore generally very closely spaced. Several wavelengths are therefore likely to be amplified by the wide spectrum amplifier medium. One thus obtains a multimode laser.
For certain applications, singlemode lasers are preferred. It is then necessary to implement a resonant optical cavity which associates a supplemental selection means with the Fabry-Perot cavity, for example by replacing one of its mirrors with a retroreflecting dispersive device.
Retroreflecting dispersive devices are commonly used in conventional optics. The most well-known device is probably the p-pitch plane diffraction grating used in the Littrow configuration.
In a general way, the dispersion plane of a p-pitch plane diffraction grating is perpendicular to its lines. A collimated light beam of wavelength .lambda., inclined at an angle .theta..sub.1 with respect to the normal of the diffraction grating which is parallel to the dispersion plane of the diffraction grating, produces a collimated beam that is also parallel to the dispersion plane and having a direction inclined at an angle .theta..sub.2 with respect to the normal, .theta..sub.1 and .theta..sub.2 being connected by the relationship: EQU p sin .theta..sub.1 +p sin .theta..sub.2 =.lambda.
The Littrow configuration in which this diffraction grating behaves as a dispersive retroreflecting system, corresponds to the case where .theta..sub.1 =.theta..sub.2 =.theta., in other words: EQU 2p sin .theta.=.lambda.
FIG. 1 shows a diffraction grating 1 implemented according to the Littrow assembly, in which an end 2 of a guided amplifier medium 3 is placed in position at the focal point of a collimating lens 4 which produces a collimated parallel beam 5 of wavelength I.
This beam is parallel to the dispersion plane of the diffraction grating, i.e. to the plane perpendicular to lines 6 of diffraction grating 1, and forms an angle .theta. with the normal 7 to the surface of diffraction grating .lambda..
In these conditions, p being the pitch of the diffraction grating, it is already known that when the relationship 2p sin .theta.=.lambda., is satisfied, diffraction grating 1 reflects beam 5 back on itself, therefore producing an image point 8 superimposed on end 2.
Such retroreflecting dispersive devices have been used, for example, to make up one of the retroreflecting systems for a resonant laser cavity in order to select one or certain beams that the cavity is able to generate. It is known for example from French patent FR-2.595.013 tunable singlemode laser sources in which the emission wavelength is selected in the wide spectrum of an amplifier waveguide with an external cavity comprising a retroreflection dispersive diffraction grating in a Littrow configuration. A movement changing the angular orientation of this dispersive device makes it possible to vary the wavelength selected and emitted by the external cavity tunable laser source.
In these different devices, the retroreflected wavelength depends on the angular orientation of a diffraction grating about an axis parallel to its lines and therefore perpendicular to the dispersion plane. This axis is called the selection axis. It is also known that such external cavity tunable laser sources can operate with what is known as a Littman-Metcalf configuration in which the incident collimated beam makes an angle .theta..sub.1 with the normal to the diffraction grating. An additional mirror is placed in position such that its normal makes an angle .theta..sub.2, on the diffraction grating. The wavelength .lambda. respecting .lambda.=p sin .theta..sub.1 +p sin .theta..sub.2 is dispersed by the diffraction grating at an angle .theta..sub.2, retroreflected on the mirror which is thus perpendicular to it, and finally, on returning, dispersed again in the diffraction grating, and is output at the input angle .theta..sub.1. This wavelength .lambda. is therefore selected in the cavity. Wavelength tunability is obtained by varying the orientation of the diffraction grating-mirror assembly, i.e. by varying .theta..sub.1, or by varying only the orientation of the mirror, i.e. by varying .theta..sub.2, or finally by varying only the orientation of the diffraction grating, i.e. by varying .theta..sub.1 and .theta..sub.2 while keeping .theta..sub.1 -.theta..sub.2 at a constant value.
FIG. 2 shows a diffraction grating 11 implemented according to the Littman-Metcalf configuration in which an end 2 of a guided amplifier medium 13 is placed at the focal point of a collimating lens 14 which produces a main collimated beam 15 of wavelength .lambda..
This beam is parallel to the dispersion plane of the diffraction grating, i.e. to the plane perpendicular to lines 16 of diffraction grating 11, and forms an angle .theta..sub.1 with the normal 17 to the surface of diffraction grating 11. By diffraction on the diffraction grating, beam 14 produces a secondary collimated beam 18 which is in the dispersion plane and forms an angle .theta..sub.2 with normal 17. A plane mirror 19 is placed in position perpendicular to beam 18 and the beam retroreflects through the whole system.
In these conditions, p being the pitch of the diffraction grating, it is already known that when the relationship p sin .theta..sub.1 +p sin .theta..sub.2 =.lambda. is satisfied, beam 15 reflects back on itself after a first diffraction on diffraction grating 11, retroreflection on mirror 19 and a second diffraction on diffraction grating 11. It therefore produces an image point 8 superimposed on end 2.
The adjustment of such devices also calls for the precise positioning of the diffraction grating about an axis perpendicular to the selection axis and parallel to the dispersion plane. This last adjustment and its stability are very delicate and govern, in most cases, the quality of the result obtained.
To provide a better understanding of this description, FIG. 3A represents a view of the focal plane of the collimating lens, showing end 2 of a guided amplifier medium and the spectrum produced in return by the assemblies in FIGS. 1 and 2 when the amplifier emits a wide spectrum. A spectrum is thus obtained which extends from a wavelength .lambda..sub.1 to a wavelength .lambda..sub.2, and for which the wavelength .lambda. is retroreflected on end 2 and therefore selected in the cavity.
In practice, since the real axis of rotation cannot be exactly parallel to the lines of the diffraction grating, the displacement of the spectrum in the focal plane will be accompanied by a movement perpendicular to said focal plane and when wavelength .lambda.' is retroreflected this will result in a configuration such as that shown in FIG. 3B in which retroreflection is not obtained exactly because of the displacement of the spectrum perpendicularly to itself, at the same time as parallel to itself in the focal plane of the collimating lens.
Consider, for example, the use of a laser diode as an amplifier medium producing a spectrum extending, in a first case, from 1470 to 1570 nm and, in a second case, from 1260 to 1340 nm, when it is desired to modify the retroreflected wavelength which then becomes .lambda.', taking into account the Littrow relationship stated above, this is achieved by rotating the diffraction grating which should, if performed about an axis parallel to lines 6 of the diffraction grating, produce a simple translation of the spectrum .lambda..sub.1, .lambda..sub.2 and bring the wavelength .lambda.' to coincide with end 2.
In the Littman-Metcalf configuration, the same problem arises when diffraction grating 11 or mirror 19, or both, undergo rotation about an axis parallel to lines 16 of the diffraction grating.
Such devices can also generate mode jumping. Indeed, rotation of the diffraction grating dispersive device changes the selected wavelength, but this wavelength must also satisfy the resonance condition applicable to all optical cavities that states that the optical length Lop of the cavity is equal to a whole number N of half wavelength: EQU L.sub.op =N.multidot..lambda./2
If the selected wavelength is lowered, the cavity must at the same time be shortened, and conversely lengthened if the wavelength increases, in order to stay on the same whole number N and avoid mode jumping.
Such a continuous tunability device without mode jumping has been proposed with a Littrow configuration (F. Favre and D. Le Guen, "82 nm of continuous tunability for an external cavity semiconductor laser", Electronics Letters, Vol.27, 183-184, [1991]), but calls for a complex mechanical assembly using two translation movements and two rotation movements.
The object of the invention is to provide a tunable laser cavity comprising a simple-to-adjust, self-aligned, retroreflecting dispersive device offering a large adjustment tolerance about an axis perpendicular to the axis of the secondary collimated beam and parallel to the dispersion plane and therefore having substantial mechanical stability.
A further object of the invention is to provide a method for displacing a self-aligned dispersive device in order to produce continuous tuning without mode jumping.
Yet a further object of the invention is to provide displacement of the self-aligned dispersive device by means of a simple rotation in order to obtain continuous tuning without mode jumping.